Avaliação do comportamento de quatro pequenos corpos lênticos em cascata em épocas de seca e chuva no semiárido.

Abstract

Small and medium size reservoirs have great importance in semiarid region for human supply, clothes washing, livestock watering and crops irrigation being important place for human activities in rural households. The quality of the accumulated water is influenced by anthropogenic activities developed in the water basin, climatic variations in climate (drought-rainfall) and hydrological factors. When distributed cascade, the upstream dams will affect those located downstream. The Experimental Basin of Sao Joao do Cariri - Paraiba State - Brazil, with 12 km2, with their nested small reservoirs and due to its small area can be used as model for larger scale studies of behavior prediction systems, both in qualitative and in quantitative terms. In this context this work aimed the study of climate factors, particularly rainfall and evaporation and its influence in some qualitative and quantitative variables of the accumulated water. The study was carried out in a nested system of small dams (Bola I, Roque, Bola III and Luiz Heleno) along the Namorado river situated in the Experimental Basin of Sao Joao do Cariri, from September 2006 to March 2009, with monthly water sampling in a central point - the limnetic zone. The water quality variables analyzed were transparency, temperature, pH, conductivity, dissolved oxygen, N-ammonia-, nitrite, nitrate, total phosphorus, orthophosphate, among others, biological chlorophyll a and phaeophytin and fecal coliform and Escherichia coli. Data from rainfall, evaporation and solar radiation were provided by staff of the Experimental Basin Meteorological Station. The results showed strong influence of climatic factors particularly the rainfall. In semiarid regions rain is concentrated in a few months (2 to 3) and sometimes alternating with dry spells but most of the year is extremely dry with no rain. The high air temperatures, intense solar radiation and evaporation have high influence on the volume of accumulated water. The dam volume at the time of water sampling was a important variable for water quality due to the influence on transparency, ions concentration or dilution, increase or decrease in salinity, electrical conductivity, hardness and macro nutrients levels, among others. Fecal conforms and Escherichia coli were present in four dams confirming the faecal contamination by homeothermic animals. This research did not show the inter-dam influence due to the absent of water flow during the rainy season and when occurred in March 2008, the streams were ephemeral, not allowing water sampling. Various trophic state indexes classified the four dams as eutrophic due to high values of total phosphorus. This parameter also classified them as Class 3 in the CONAMA Resolution 357/2004, which establishes its suitability for domestic supply after conventional or advanced treatment, for crops irrigation those not consumed raw and for watering live stock .